Introduction

“For every affliction of the mind that is attended with either pain or pleasure, hope or fear, is the cause of an agitation whose influence extends to the heart.”
—William Harvey

It is no secret that aerobic exercise is good for your heart. Aerobic exercise eventually leads to a lower resting heart rate, which can be indicative of a strong heart, which can lead to reduced heart problems.

Seems simple. However, the underlying mechanisms contributing to a lower resting heart rate are not so simple. The heart-brain connection is incredibly complex. Something as seemingly simple as your heart beat has numerous systems at work modulating how well it performs. In this article, I’m going to discuss some of these mechanisms so that you have a basic understanding of what determines how fast (or slow) your heart beats, why this is important to understand, and then delve into a discussion of heart rate variability and how you can use this to evaluate both your recovery after a workout, and how well your body responds to stress.

What Makes Your Heart Beat?

A lot of things, actually. But before we get into that, let’s look at some basic cardiac anatomy and what your heart does:

“The muscular heart consists of two atria and two ventricles. The atria are upper receiving chambers for returning venous blood. The ventricles comprise most of the heart’s volume, lie below the atria, and pump blood from the heart into the lungs and arteries. Deoxygenated blood enters the right atrium, flows into the right ventricle, and is pumped to the lungs via the pulmonary arteries, where wastes are removed and oxygen is replaced. Oxygenated blood is transported through the pulmonary veins to the left atrium and enters the left ventricle. When the left ventricle contracts, blood is ejected through the aorta to the arterial system.”1

Your heart is responsible for moving the entire volume of your body’s blood through the lungs where it eliminates waste and replenishes oxygen, then sends it all back out through the body. You can imagine how important it is to keep this vessel beating.

The Intrinsic Cardiac Nervous System

Your beating heart has a self-contained nervous system, complete and complex enough to be referred to as a mini brain. Two internal pacemakers keep it pumping and can operate on their own under certain conditions:

“The heart contains autorhythmic cells that spontaneously generate the pacemaker potentials that initiate cardiac contractions. These cells continue to initiate heartbeats after surgeons sever all efferent cardiac nerves and remove a heart from the chest cavity for transplantation. Autorhythmic cells function as pacemakers and provide a conduction pathway for pacemaker potentials.
The sinoatrial (SA) node and atrioventricular (AV) node are the two internal pacemakers that are primarily responsible for initiating the heartbeat.”1

The primary pacemaker of the heart is the sinoatrial node, which sends electrical signals throughout your heart’s intrinsic nervous system, causing the heart muscles, or myocardium, to contract.

The secondary pacemaker, the atrioventricular node, works in tandem with the first. Your sinoatrial node fires, sending an electric impulse to the atrioventricular node, causing it to fire as well. This is where the thump-thump beat of your heart comes from —the atria, or top two chambers of your heart, contract when the sinoatrial node fires, followed by the contraction of the ventricles, or the bottom two chambers, when the atrioventricular node fires. The system is entirely self-contained and can continue to operate for a time once severed from the body. In the event that the sinoatrial node is damaged, the atrioventricular node can take over, though it’s not as strong, and won’t last as long in the driver’s seat.

This is a simplified overview of your heart’s internal pacemaking system. Simple enough on it’s own, it gets complicated very quickly when we take the rest of the body into account.

Autonomic Regulation of the Heart

Though your heart has a self-contained system and can run on it’s own if necessary, it does connect to the rest of your nervous system via its extrinsic nervous system and is modified by it. This is a good thing. The sinoatrial node, without regulation, will run anywhere from 60 to 100 beats per minute (bpm). “The [sinoatrial] node’s intrinsic firing rate of about 60–100 action potentials per minute usually prevents slower parts of the conduction system and myocardium (heart muscle) from generating competing potentials.”1

A healthy heart generally beats anywhere from 55 to 75 bpm. The reason for this is that the sinoatrial node —along with the rest of your heart— receives inputs from both the sympathetic and parasympathetic branches of the autonomic nervous system.2

“The extrinsic cardiac [autonomic nervous system] may be subdivided into sympathetic and parasympathetic components. The sympathetic fibers are largely derived from major autonomic ganglia along the cervical and thoracic spinal cord. … These ganglia house the cell bodies of most postganglionic sympathetic neurons whose axons form the superior, middle, and inferior cardiac nerves and terminate on the surface of the heart. The parasympathetic innervation … are carried almost entirely within the vagus nerve and are divided into superior, middle, and inferior branches. Most of the vagal nerve fibers converge at a distinct fat pad between the superior vena cava and the aorta (known as the third fat pad) en route to the sinus and atrioventricular nodes.”3

When you’re at rest, the parasympathetic branch is typically active, and the vagus nerve delivers inhibitory signals to the sinoatrial and atrioventricular nodes, causing them to fire more slowly. We’ll refer to this as the vagal brake. When you’re active – exercising, for example – the sympathetic branch inhibits the parasympathetic signals to these nodes, lifting the brake and allowing the heart to beat at a faster rate. This is known as “accentuated antagonism” and describes the “enhanced negative chronotropic effect of vagal stimulation in the presence of background sympathetic stimulation.”1

Because of the way the autonomic nervous system modulates the frequency of your heart’s activity, your heart rate can be used to measure autonomic balance and determine parasympathetic activity to some extent. Generally, the lower your resting heart rate, the greater your vagal tone, which in turn is indicative of lower stress. Which brings us to why it’s important to understand this.

Stress affects everyone in different ways. While your stress response is activated during exercise or under threat of disease, injury, or death, this activation is typically temporary and useful to your survival and well-being. Chronic stress, however, is harmful in the long run – your sympathetic nervous system is not designed to run at full activation all the time, and several physiological systems suffer as a result of prolonged activation. More important to our longevity than the stress that induces action is the recovery: if your body does not return to a state where your autonomic nervous system is in balance, and quickly, you’re in trouble.

“If the parasympathetic nervous system governs your heart rate at rest, and it goes down as a result of training, we can surmise from that that you have improved your parasympathetic nervous system. And that’s exactly what we do see in studies of heart rate variability and exercise. Exercise improves your parasympathetic nervous system function. It’s your parasympathetic nervous system that largely governs your ability to recover …”5

Specifically mentioned is heart rate variability, not heart rate, in these studies that determine the improvement of the parasympathetic nervous system. While heart rate will tell you a bit about heart health and autonomic function, it does not provide you enough information to determine how well your nervous system recovers from stress. This is where Heart Rate Variability comes in. Before we get to that, however, let’s look at systems in place that both create and help you understand this phenomenon.

Respiratory Sinus Arrhythmia

So, by now you should see how your parasympathetic and sympathetic nervous systems modulate your heart rate on a large scale – at rest or during exercise. This modulation, however, is not strictly limited to larger time frames or specific stressors. This modulation occurs in an ongoing fashion, and is most frequently activated by something as simple as your breath. This is called respiratory sinus arrhythmia. Respiratory, of course, having to do with respiration, or breathing; sinus, as in affecting the sinoatrial node; and arrhythmia, to describe the irregular rhythm of your heart beat:

“Breathing oscillations influence the firing activity of the sinoatrial node defined as respiratory sinus arrhythmia (RSA). RSA is a consequence of various central as well as peripheral effects from medullary cardiorespiratory centre and reflex responses from pulmonary/cardiovascular receptors resulting in heart rate oscillations in accordance with respiratory cycle.”6

When you inhale, the autonomic balance tips toward the sympathetic nervous system, inhibiting parasympathetic signals to the sinoatrial node, causing your heart to beat faster. When you exhale, sympathetic inhibition lifts, the balance shifts in favor of the parasympathetic system, and the vagal brake is applied to the heart, slowing your heart rate. This is why you’ll hear trainers and yoga instructors often cueing a longer exhale: this elicits a recover response from the body.

Generally, this happens automatically, with little voluntary control: “we do not normally think about our breathing. It is automatic; our breathing depth and rate varies without our conscious awareness due to changes in the inputs to the respiratory centers in the brain stem that control respiration.” However, you are able to consciously speed up or slow down your breathing rate: “Since we have conscious control over our breathing, cognitively-directed breathing exercises can be used to impose a breathing rhythm on the heart rhythms.” Respiratory sinus arrhythmia, then, is where you can begin to not only understand heart rate variability, but also learn how to exert some voluntary control over your heart.7

What the Hell is Heart Rate Variability?

As simply as I can sum this up, heart rate variability is the average measurement of the differences in time between each heart beat.

As you have seen above, your heart rate changes on a beat-to-beat basis – called the “interbeat interval” – modulated by your autonomic nervous system, based on your breathing rate, among other factors: “The sympathovagal interaction from the [central autonomic network] results in multiple instantaneous heart rate variations, i.e. the heart rate variability.”6

Heart rate variability is scored in a number of ways, though I’ll not dig into the details surrounding the measurement process here. References to various studies are provided below if you’re interested in understanding how this is measured. Suffice it to say that different programs will use different means of acquiring the score. What will be most useful to you is how this information can be beneficial.

Knowing what systems are in place that modify heart rate in an ongoing basis makes it possible to determine, to some extent, how well particular systems in your body function. I have only touched on respiratory sinus arrhythmia, and autonomic control of the heart. What you want to consider is that your autonomic nervous system connects to other systems as well, modifying those systems or receiving inputs from those systems to modify others. Including the heart. Because of this, heart rate variability “can be used as an index of the functional capacity of various regulatory systems”7 and can be “considered a measure of neurocardiac function that reflects heart–brain interactions and [autonomic nervous system] dynamics.”1 To understand this phenomenon a little more clearly, let’s look at the psychophysiological responsiveness, or “reactivity,” of the heart to stress.

Reactivity Hypothesis

We’ve discussed some of the underlying mechanisms of heart rate variability, so you can begin to understand why this measurement might be a powerful indicator of heart-related health. Because there are various systems in place that can modify heart rate variability, this indicator can extend beyond the heart, making it possible to determine other things going in your body in terms of illness and recovery. Researchers have developed the reactivity hypothesis, “which proposes that cardiovascular responses to a stressor may be predictive of certain diseases, as well as useful in monitoring the training status of high performance athletes.”4

Physiologically, this is fantastic, because we now have an indicator to preemptively understand if something detrimental is occurring in the body. While you might feel well, a heart rate variability measurement may suggest that your body is dealing with something internally, i.e. maybe you’re coming down with a cold, or you’re bordering on heart failure of some sort:

“Heart rate variability analysis seems to be an attractive, noninvasive method to study cardiac autonomic activity. Power spectral analysis of heart rate variability over a period of ECG recordings to reflect cardiac sympathetic tone or sympathovagal balance has been widely used. … These analyses provide significant prognostic value in that a depressed heart rate variability or baroreflex sensitivity after myocardial infarction is associated with higher cardiac mortality.“3

Heart rate variability can indicate the onset of heart failure, particularly after a heart attack. This is incredibly useful in preemptively addressing heart problems before they become too far advanced to deal with. Consider this: a transplanted heart does not integrate with the new host body’s autonomic nervous system, which means there is no vagal brake applied to the sinoatrial node of the incoming heart. Without parasympathetic innervation and modulation, the transplanted heart will wear out more quickly than your original, well-cared for heart. In terms of longevity, it makes sense to be aware of your heart’s condition to reduce your chances of needing a heart transplant.

Avoiding a heart transplant would be grand. Avoiding any surgery altogether would be better. As mentioned before, heart rate variability provides the means by which to understand the health or oncoming failure of other systems:

“… this [central autonomic] network is an integrated system for internal self-regulation by which the brain controls the heart, other visceromotor organs, and neuroendocrine and behavioral responses that are critical for goal-directed behavior, adaptability, and sustained health. … these dynamic connections explain why vagally mediated [heart rate variability] is linked to higher-level executive functions and reflects the functional capacity of the brain structures that support working memory and emotional and physiological self-regulation.”7

Furthermore, “reduced physiological regulatory capacity may contribute to functional gastrointestinal disorders, inflammation, and hypertension.”7 Putting this all together suggests that by understanding your own heart rate variability, you can have an idea about physiological self-regulation and determine if something is at odds in your body. Is your heart rate highly variable? If not, what does it mean to see lower, or depressed, variability? And if you’re training, how can you use heart rate variability to your advantage?

The Training Edge

Have you ever done a workout and felt sluggish, or off, about it? It could be due to what you’ve eaten. It may also be due to your body’s cardiac and autonomic recovery. However, without a way to measure sympathovagal balance or autonomic recovery, there’s no way to tell. Heart rate variability provides a way to figure out if you’re ready to take on another grueling training load, based on how well your nervous system has recovered from the last bout.

“Most [heart rate variability] measures are substantially reduced during exercise … [heart rate variability] has also been employed as a tool to investigate post-exercise autonomic (predominantly parasympathetic) activity. Upon exercise cessation, [heart rate] and [heart rate variability] demonstrate a time-dependent recovery and eventual return to pre-exercise levels. … Rapid (though incomplete) recovery is commonly observed in the initial minutes following exercise. … complete recovery may take up to 48 h following some bouts of exercise and may sometimes involve an “overshoot” above pre-exercise levels prior to 48 h …”4

We know that delayed onset muscle soreness, otherwise known popularly as DOMS, can take effect anytime within a 48 hour period. This is a mechanical response to training, and one in which you are clearly made aware of: you hit the squat rack one day, and two days later you have trouble sitting down to poop. Your sore muscles, however, tell you nothing about your heart. After a tough workout, your heart rate quickly returns to normal levels, but this doesn’t tell you if your heart —and, by extension, your nervous system— has recovered completely from your training session: “a higher exercise intensity is associated with a slower recovery of [cardiac parasympathetic neural activity-heart rate variability] measures.”4 With heart rate variability, you’ll have a better idea of where your body lies in terms of recovery.

Tracking Heart Rate Variability

By now you should have a basic understanding of how useful tracking your heart rate variability may be. Its popularity continues to grow as more and more organizations utilize this metric to improve disease and organ failure prevention, therapy, and increase training effectiveness. Strvtmvmnt utilizes heart rate variability to monitor clients and address day to day fluctuations to maximize their training potential. Which leads us to the question: how, as a consumer, can I track this?

Heart rate variability is still in its infancy among consumer products. However, apps such as Apple Health can take a measurement of heart rate variability when linked to a heart rate monitor or other app capable of acquiring this information. A basic Google search will pull up several apps available across the App Store and Google Play, and more apps and devices are in development to take more accurate measurements and provide better feedback regarding the data they receive from you.

My guess is that it won’t be long before your primary care physician begins tracking this as well, monitoring your health from afar, and providing notice when something looks off. The better information we can gather about our own health and recovery, the more productively we can address issues that arise before we are consciously made aware of them, i.e. by pain or other unwelcome symptoms.

Take the time to do a little homework and find an app or program that works for you. If you’re interested in training with someone who will do the work for you regarding heart rate variability tracking, get in touch with me via the contact form linked above, or via any social platform on which I am present.

Your health is your responsibility, but you don’t have to figure it all out on your own. There are many resources becoming available to help navigate these complicated waters. This does, however, still require effort on your part. If you want to do the best for yourself, you must get moving.

Strength and Mobility.
I offer quality content and programs to promote the mental fortitude and physical health of my clients, students, and followers. The information on this site and across strvtmvmnt’s media channels is aimed toward this goal, with a focus on the ability to apply this knowledge with no additional equipment necessary but your own body.